1. Field of the Invention
The present invention relates to a microencapsulation shell system which provides for the controlled release of the material encapsulated thereby. Microcapsules, methods of manufacture, and methods of using the microcapsules are particularly contemplated by the invention.
2. Description of Related Art
Timing is everything in many technologies. That is particularly true for chemically-based technologies such as agriculture, insecticides, and fungicides where contact or inadvertent ingestion may pose serious health risks to humans and animals. Paraquat is one example of a useful chemical that poses a serious health risk due to inadvertent ingestion, inhalation, or contact. Such chemicals should be enclosed and protected from at least the point of manufacture, through transport, and until the chemical is loaded into some form of wet or dry product applicator. Because such contact has been generally unavoidable for most chemical products, a complex scheme of regulation has been established to control the handling and exposure risks to humans as well as nontarget animals. The art has faced a long standing need for a means of preventing contact between useful but potentially hazardous chemicals and nontarget organisms without reducing the efficacy of the chemical agent.
Another situation where timing is important is where chemically active agents must maintain an extended presence to be effective or must migrate from an applied position to a more desired location. An example of such a chemical is the soil insecticide diazinon which is a contact pesticide that used to be available for application to sod farms and golf courses in the form of solid granules on a corn cob grit carrier. Contact with water would dissolve the granule from the carrier and wash the pesticide into the soil. Diazinon was quite effective at controlling soil-borne pests that would otherwise damage the grass.
Unfortunately, the diazinon-containing granules were applied during the day and exhibited a particle size that was large enough to sit on top of the turf thatch. Birds flying overhead in search of food could still see the granules and would mistakenly them as food. The Environmental Protection Agency recently prohibited further use of diazinon for sod farms and golf courses due to the unacceptably high level of avian mortality from these uses. It would be desirable to have a means for applying granulated contact insecticides during normal daylight hours with protection against avian feeding but yet be able to release the insecticide when avian feeding does not pose a risk of mortality due to accidental consumption.
One possible approach suggested by Michael U.S. Pat. No. 4,946,624 for laundry products is a crosslinked glutaraldehyde microcapsule having a core material, preferably perfume, surrounded by a capsule shell wall of coacervated gelatin and gum arabic that have been crosslinked with glutaraldehyde. This shell wall can have small "particles" of 0.1-25 % of the core diameter that can be "activated" with heat from the drier or warm water from the wash or rinse cycle in a clothes washing machine to form discrete holes in the shell. The particles can be liquids that will volatilize from heat or solids that will dissolve in wash or rinse water to form pores or holes through which the hydrophobic core material escapes. No specific list of solid inclusion particles is provided. Perfumes, flavors, pharmaceutical materials and agricultural chemicals in general are taught to be useful for the encapsulation.
In many applications, however, the core material must be and is desirably hydrophobic. The wall of the microcapsule must be able to form porosity in ambient or cold water and do so without affecting the viscosity of the solution or leave residues that could clog spray nozzles or tubing.
In other applications, microcapsule walls made of gelatin and gum arabic crosslinked with glutaraldehyde are not sufficiently impermeable to restrain the core material and remain compatible with a water carrier. This is particularly true for some hydrophobic materials including herbicides and plant growth regulating agents, insecticides, and fungicities that have a particularly strong solvation ability. Some form of better encapsulation system is needed to form such materials into microcapsules that will be impermeable to the core material.
It would be desirable to have a microencapsulated system that could be used for herbicides and plant growth regulating agents, insecticides, and fungicides without modification of the handling, mixing, or application methods currently in use.
It would also be useful to have a microencapsulation system that would encapsulate hydrophilic materials as well as a broad spectrum of hydrophobic materials including those of high salvation ability yet permit release of the encapsulated material upon contact with water having a temperature of less than about 100.degree. F. (38.degree. C.).